High strength, corrosion resistant, austenite-ferrite stainless steel

ABSTRACT

An aging, austenite-ferritic, stainless steel having very high strength, good ductility and notch-impact resistance as well as excellent resistance to general and intercrystalline corrosion, has the following composition, given in percent, by weight: C maximum value 0.15 preferably 0.01 - 0.10 Si maximum value 2 preferably a maximum of 1.0 Mn maximum value 5 preferably a maximum of 2.0 Cr 20 - 30 preferably 24 - 27 Ni 4 - 9 preferably 5.5 - 7.5 Mo 1 - 3 preferably 1.3 - 1.8 Nb 0.1 - 2.5 preferably 0.1 - 1.6 Ti maximum value 1.5 preferably a maximum of 1.0 Al 0.1 - 2 preferably 0.5 - 1.0 N maximum value 0.15 preferably a maximum of 0.1 THE REMAINDER BEING MADE UP OF IRON AND THE IMPURITIES AND OTHER MATERIAL USUALLY PRESENT IN THIS TYPE OF STEEL. It is further necessary to weigh out the quantity of niobium and titanium in proportion to the quantity of carbon and nitrogen in such manner that the atomic percentage of niobium plus the atomic percentage of titanium &gt; OR = the atomic percentage of carbon plus the atomic percentage of nitrogen.

United States Patent 1 Hellner et al.

[ Feb. 11, 1975 HIGH STRENGTH, CORROSION RESISTANT, AUSTENITE-FERRITE STAINLESS STEEL [73] Assignee: -AB Boi'ors, Bofors, Sweden [22] Filed: Apr. 9, 1973 [211 Appl. No.: 349,359

[30] Foreign Application Priority Data Apr. 24, 1972 Sweden 5353/72 [52] US. Cl 148/37, 75/124, 75/128 C, 75/128 W [51] Int. Cl. C22c 39/14, C22c 39/26 [58] Field of Search 75/124, 137, 128 G, 128 W [56] References Cited UNITED STATES PATENTS 3,152,934 10/1964 Lula 75/124 3,253,908 5/1966 Tanczyn 75/124 3,328,211 6/1967 Nakamura 75/124 3,362,813 1/1968 Ziolkowski 75/124 3,759,757 9/1973 Primary ExaminerHyland Bizot Attorney, Agent, or Firm-Elliott I. Pollock [57] ABSTRACT An aging, austenite-ferritic, stainless steel having very high strength, good ductility and notch-impact resistance as well as excellent resistance to general and intercrystalline corrosion, has the following composition, given in percent, by weight:

C maximum value 0.15 preferably 0.01 0.10

Si maximum value 2 preferably a maximum of 1.0 Mn maximum value 5 preferably a maximum 01'10 Cr 20 30 preferably 24 27 Ni 4 9 preferably 5.5 7.5

Mo 1 3 preferably 1.3 1.8

Nb 0.1 2 5 preferably 0.1 1.6

Ti maximum value 1 5 preferably a maximum of 1.0

0 1 2 preferably 0.5 1.0

preferably a maximum of 0.1

the remainder being made up of iron and the impurities and other material usually present in this type of steel. It is further necessary to weigh out the quantity of niobium and titanium in proportion to the quantity of carbon and nitrogen in such manner that the atomic percentage of niobium plus the atomic percentage of titanium z the atomic percentage of carbon plus the atomic percentage of nitrogen.

6 Claims, No Drawings HIGH STRENGTH, CORROSION RESISTANT, AUSTENITE-FERRITE STAINLESS STEEL BACKGROUND OF THE INVENTION It is a characteristic of an austenite-ferritic steel of high chromium content that the steel possesses excellent corrosion resistance even in highly corrosive environments. One example of such a steel, known as type SIS 2324, contains approximately 0.1% C, 26% Cr, 5% Ni and 1.5% Mo. This steel is used in many applications where the requirements for corrosion resistance are very demanding, and this type of steel also possesses high strength in comparison with regular austenitic, stainless steels. However, there is a definite need for a steel of still greater strength than is possessed by the type SIS 2324 steel, and specifically in combination with good ductility and notch-impact resistance as well ashigh resistance to corrosion.

A method commonly employed to attain increases in steel strength is age-hardening. This requires a suitable combination of alloy material as well as an appropriate aging treatment in order to attain the fine-dispersed precipitate which is necessary for such increase in strength. The aging treatment, which is usually accomplished at relatively low temperatures, is preceded in most cases by a solution anneal at high temperature. However. in the case of austenite-ferritic steels having a high chromium content. such as the above identified type SIS 2324 steel. it has been found that such agehardening cannot be accomplished satisfactorily. 1f the aging treatment is carried out at temperatures between 400C and 525C, there will occur the well-known socalled 475C-embrittlement. Likewise, if the aging treatment is carried out at temperatures between 700C and 850C, it results in an embrittlement by '-phase precipitation. It follows, therefore, that the above-mentioned temperature ranges can never be used to carry out the aging treatment, because the impact strength of the steel then becomes unacceptably low.

An aging treatment at temperatures ranging from 525C to 700C is not practical either because it has been found that, in case of a treatment at such temperatures, the susceptibility to intercrystalline corrosion becomes unacceptably great. As described in prior US. patent application Ser. No. 823,744 and US. Pat. No. 3,717,455 it is possible to reduce substantially the adverse tendency of intercrystalline corrosion in case of steels of this kind by the admixture of niobium and titanium. However, tests conducted with steels of type SIS 2324, employing the admixture of strong carbideforming substances, for example in the form of niobium or titanium, did not result in any increase in strength when subjected to aging treatment within the specific temperature range from 525C to 700C.

The above discussed facts indicate clearly that it is not possible to attain in a conventional manner, by admixture of alloys and age-hardening, and increase in strength for steels of the type SIS 2324 if the requirements for good impact strength and good resistance to intercrystalline corrosion are to be maintained. Such age-hardening would accordingly seem to require an aging treatment at temperatures other than the unsuitable temperature ranges discussed above, i.e., at temperatures either below 400C or above 850c. However, at temperatures below 400C the rate of diffusion of the material is extremely low and, as a result, any precipitation hardening progresses at such slow speed that it can never be carried to completion in practice. On the other hand, if temperatures above 850C are employed, the rate of diffusion becomes so high that overaging takes place very rapidly. thus making a proper age-hardening practically impossible in the present state of the art. Therefore, it is considered to be definitelyimpossible to increase the strength char acteristics of steels of type SIS 2324 by means of agehardening, while maintaining at the same time the impact strength and resistance to corrosion of said steel.

SUMMARY OF THE INVENTION The present invention makes it now feasible in the case of a corrosion'resistant austenite-ferritic steel having high strength, good ductility, impact strength and resistance to intercrystalline corrosion, surprisingly to attain, by means of age-hardening, an increase in its strength and resistance to corrosion while maintaining its impact strength. This is accomplished by combining and adding a powerfully carbide-forming compound, such as niobium and titanium as well as aluminum, to an austenite-ferritic steel having a high content of chromium.

In order to attain the desired strength characteristics, special attention must be paid to the proportion of austenite to ferrite in the steel. Excessive quantities ofaustenite in the steel will cause a lowering in strength after age-hardening, and tests have shown that the content of austenite must accordingly not exceed 35% by volume. On the other hand, the austenite influences ad vantageously the impact resistance when the material possesses the desired high strength characteristics, and the content of austenite should therefore not be reduced to a very low level. Thus, the austenite content will be a factor for attaining a fine ferrite grain size by its retarding effect on the growth of the ferrite grains during the solution anneal. Furthermore, the austenite phase has an inhibiting influence on the propagation of cracks, and thus influences advantageously the ductility and the impact resistance of the steel. Tests have shown that, in view of the above discussed factors, the austenite content of the steel must be at least 10% by volume for the purpose of the present invention.

As a result, in order to attain the very advantageous combination of high strength and good ductility, which is a characteristic of this invention, the steel of the present invention must contain between 10% and 35%, and preferably between 10% and 25%, by volume of austenite. In order to assure that the steel has an austenite content within these specified limits, it is necessary to take into consideration the ferriteor ausenitestabilizing effect of the alloys contained in the steel, and this can be accomplished roughly with the aid of a so-called Schaeffler diagram.

The invention is further characterized by the fact that the steel has the following composition, given in percent by weight:

preferably 0.0l 0.10

preferably a maximum of 10 preferably a maximum of 2.0

C maximum value 0.15 Si maximum value 2 Mn maximum value 5 Cr 20 30 preferably 24 27 Ni 4 9 preferably 5.5 7.5

Mo 1 3 preferably 1.3 1.8

Ti maximum value 1.5 preferably a maximum of 1.0 A1 0.1 2 preferably 0.5 1.0

preferably a maximum of 0.1

bium and titanium in proportion to the quantity of carbon and nitrogen in suchmanner that the atomic percentage of niobium plus the atomic percentage of titanium the atomic percentage of carbon plus the All steeltypes were subjected to a 1-hour solution treatment'at 1000C and an aging treatment with the exception of steel No. 1 (standard steel SIS 2324) which was not subjected to the aging treatment but atom": percentage. nitrogen only to a normal quenching w1th water cooling. Natu- Powerfully carblde-formmg substances other than rally, the reason for thls actlon 1s due to the fact, emn1ob1um and titanium, for example zirconlum and tantalum, can also be part of the alloy and, obviously in phaslzed prevlously h an aglflg ue atmen t for F this case, should be u li d i ffi i i hi standard steel SIS 2324 is not feasible without incurring ric amounts in proportion to carbon and nitrogen. The an unacceptable deterioration of the impact strength or contents of carbon and nitrogen should be kept so low the resistance to intercrystalline corosion. as p i n 3 lowest value f ppr ima ly Tables 1 and 2 show that even in case of small admix- Percem was found to be proper m PTaCtICe' tures of aluminum, such as 0.1%, significant increases EXAMPLES in strength are obtained after the aging treatment. As The invention will now be further explained by refer- Stated a tests confjucted wljth corrlfsponsmg Stze] ring to several types of steel, whose chemical composityp es the alummufn adm'xture ave 5 Own at tions are li ted i Tabl 1 b l an 1ncrease 1n strength W111 not occur after concluslon Table 1 Chemical Composition in by Weight Steel No. C Si Mn Cr Ni Mo Nb Al N Steel No. 1 in Table 1 above is the type 818 2324 steel discussed earlier. Steels 7 and 8 are type SIS 2324 with l of the aging treatment. These facts are therefore an inniobium only, or with aluminum only, respectively,

added thereto. The other types of steel (Nos. 2-6 inclusive) in Table 1 are examples of the present invention, and the steel types 4, 5 and 6 having aluminum contents ranging from 0.5% to 1.0% are especially characteristic of the invention.

Table 2 (on the following page) lists the mechanical properties of the steel types shown in Table 1, together with hardness and aging treatment.

Table 2 Tensile Strength. Notch-Impact Resistance, Charpy-U and Hardness After Solution Anneal at 1000C, One Hour, Water Cooling and Aging as Stated Below 0.27: PS UTS Elongation Reduction of Notch Impact Austenite Steel 5 D Area Con- Strength Content No. kg/mm kg/mm (Expansion traction kgm/cm HV by Volume Aging Treatment 1 51.0 64.3 27.0 60.4 9.7 230 26 None 2 66.7 80.6 22.0 56.0 8.4 282 18 575C 1h, water cooling 3 69.6 80.6 19.5 56.0 7.8 290 15 575C lh, water cooling 4 83.6 92.5 18.0 59.4 7.9 32 1 13 575C 1h. water cooling 5 76.6 89.5 20.5 56.1 8.2 313 24 575C 111, water cooling 6 91.5 99.5 14.5 43.8 4.6 356 10 575C 1h. water cooling 7 53.7 68.6 23.0 61.0 8.0 244 23 575C lh. water cooling 8 85.6 94.5 17.0 47.4 5.5 302 10 575C 1h. water cooling 2 63.7 79.6 22.5 56.0 8.2 268 18 600C lh, water cooling 3 66.7 81.6 20.5 56.0 7.5 274 15 600C lh, water cooling 4 75.6 86.6 20.0 61.0 8.2 303 13 600C 1h, water cooling 5 69.6 84.6 21.5 f 61.0 8.7 280 24 600C lh, water cooling 6 86.6 97.5 13.5 49.2 6.6 323 10 600C 1h, water cooling 7 52.7 67.6 23.0 61.0 7.6 236 21 600C 1h, water cooling 8 76.6 86.6 19.0 56.1 6.7 274 10 600C 1h. water cooling Table 3 Temper and Austenite Content After a Mere Annealing In Solution at l000C, One Hour. Water Cooling According to Table 5 the standard steel SlS 2324 shows a substantial loss of weight after the aging treatment (at 600C, for 1 hour, with water cooling), while steel No. 8 (SlS 2324 aluminum) shows a much lesser loss St I HV cc z i i g gfigf'; 5 after an ldentlcal aging treatment according to Table 4. Steels 4, 5 and 6, also listed in Table 4, prove definitely gig fig that a combination of aluminum with niobium in steel 3 237 5 SIS 2324 will eliminate completely the susceptibility to 4 237 13 5 235 24 IO corroslon. I 6 254 10 As prevlously emphasized, the standard steel SlS i g 2324 becomes susceptible to intercrystalline corrosion after aging treatments at temperatures ranging approxi- A comparison f Tab|es 2 and 3 proves that the high mately from 500C to 750C. This is the caseeven after strength characteristics of the steel types are definitely Short Perlods of agmg and especlally qulck at a due to the aging treatment because the tables show that temperature around 60000 Obvlouslyi Such susceptlthe steel types have a much l w h d after b i bility to intercrystalline corrosion is particularly trousubjected only to solution anneal, and that the hardness bleSOme and definitely unacceptable for y p after a mere solution ne l Shown b T bl 3) i tant fields of applicatlon, for example ln centrlfugal fully comparable with the typical hardness which is ob- Separators Susceptibility intercrystalline Corrosion tained for the standard steel SIS 23 24, St d d was examined for the steel types listed in Table l, with strength for the steel type SlS 2324 is approximately 50 the tests being carried out under the high strength conkg/mm 0.2% proof stress. It is apparent that it will be ditions shown by Table 3, i.e., a 1-hour heat solution necessary to admix aluminum with the steel types, as treatment at 1000C, followed by water cooling, and proposed by the present invention, in order to attain 25 then followed by an aging treatment at 575C for l the desired increases in strength after aging treatment hour and water cooling, or followed by an aging treatbecause the standard steel SlS 2324 does not show any ment at 600C for 1 hour and water cooling respecincrease in strength after aging treatment at 575 to tively. Corrosion tests were then conducted by using 600C disks (3 X 20 mm diameter) which were subjected for It was found that alumlnum contents between 0.5 20 hours to a boiling 1% solution of sodium chloride, and 10% are most Sultable for anamlng maXlmUm saturated with finely powdered silver chloride and calcl'eases m Strength y age-hardemng- Such aluminum cium hydroxide. This method was found to be very ef Contents, furthermore, have an advantageous Influence fective to detect any susceptibility to intercrystalline on the steels resistance to corrosion, another feature Cormsion i hl i d di i h Case f steels of of the invention, an influence which is demonstrated type 515 2324 Clearly by tables 4 and 5 below" The corrosion test results are listed in Table 4 above.

Table 4 Steel No. 2 suffered some loss of weight but the corrow i'r' c 's,ad'trcr Results of Corrosion Tests (20 Hours in Boiling l "/1 S.|0ns erepr md lly Spot Orroslof n m e I ystal NaCI-Water Solution. Saturated with AgCl. and Ca(OH)-,). 40 lme corroslons were rare- The I055 of weight was Conditions of Heat 'lre atlnellt: Solution Anneah stamially lower f r steel N0, 3 than in the case of steel g gg gg lg wmcrumlmg No. 2, and spot corrosions occurred only in isolated L f O O A cases. Steels 4, 5 and 6 did not suffer any measurable 055 O CCUH'QI'ICC CCUI'ICHCC glng weight in of SP0 onmemys Tm loss of weight, and no corroslons were found at all. Steel (Msdifln corrosions wlline COHOSIOHS ment The results of slmllar tests, conducted with standard Val) steel SIS 2324, are listed in Table 5 above. When this i 3.9 throughout insignificant 575C In standard steel S15 2324 was tested by the above de- .2 in spots none water 4 0400 one one cooling scr bed method, aglng treatments were conducted over 5 0.00 none none varlous perlods of tlme at approxlamately 600 C prlor 6 9 to the corrosion treatment. Even short-time aging treat- 7 3.9 throughout in spots 3 Q4 in spms in Spots ments, such as 5 minutes, caused a very large loss of 2 8-88 600C weight, e.g., almost 10%, and the loss became greater 6 23:: 28:: 253m still when the time of treatment was lengthened. Fur- 4.4 throughout in spots thermore, when the standard steel S15 2324 was sub- 8 Spms 5pm jected to aging treatments, spot corrosions and very ex- Table 5 Results of Corrosion Tests for a Steel No. 1 (S15 2324) (20 Hours in :1 Boiling 1% NaCl-Water Solution, Saturated With AgCl, and Ca(OH)- Conditions of Heat Treatment: Quenching at l000C for one Hour. Water Cooling and Aging as Stated Below.

Loss of Weight Occurrence of Steel Aging ln Percentage Spot Corlntcrcrystal- No. Treatment (Median Value) rosions line Corrosion 1 None 0.00 None None I 600C, 5 minutes.

water cooling 9.5 throughout throughout 1 600C. l5 minutes.

water cooling l0.9 throughout throughout 1 600C. 1 hour.

water cooling 12.0 throughout throughout tensive intercrystalline corrosions occurred abundantly. However, if solution treatment at 1000C for 1 hour, followed by water cooling, was used solely, steel type SIS 2324 did not suffer any measurable loss of weight and there was no corrosion at all.

The above tests demonstrate that steels composed in accordance with the present invention become, in spite of being aged to increase their strength characterisics, very resistant to general as well as intercrystalline corrosion. The corrosion resistance is fully comparable to the corrosion resistance of standard SIS 2324 steel in its merely quenched state, after the standard steel has been cooled down effectively from the solution temperature. If, however, the standard steel SIS 2324 is used in large dimensions, as is often the case, the rate of cooling throughout the critical temperature range will become so slow that the steel becomes susceptible to intercrystalline corrosion even after quenching. The steels of this invention are completely indifferent to this slow cooling-off process, and retain their resistance to intercrystalline corrosion even if these steels are employed in large dimensions.

The above test results demonstrate that the present invention makes it now feasible to produce a stainless, ferrite-austenitic steel of very high strength, good ductility and notch-impact resistance and excellent resistance to general as well as intercrystalline corrosion. This combination of characteristics was impossible to attain heretofore for steels of the kind here involved.

The steel of the present invention is particularly suitable for use in centrifugal separators and other rotating machine units which operate in hot chloride solutions or which come occasionally in contact with such solutions. Other fields of application are, for example, pump shafts, gear shafts, drive shafts for boat-engines, bolts, stirring equipment, and transport devices for the chemical industry and the cellulose industry, as well as any other parts which are subjected to high stresses in corrosive surroundings, especially if there is a danger of intercrystalline corrosion.

We claim:

1. A corrosion-resistant, austenite-ferritic steel which contains to by volume of austenite, said steel being adapted to attain an increase in strength by agehardening, while retaining good ductility and notchimpact resistance as well as excellent resistance to intercrystalline corrosion, said steel consisting essentially of the following composition by weight:

C, a maximum of 0.15% Si, a maximum of 2% Mn, a maximum of 5% Cr, from 20% to 30% Ni, from 4% to 9% the remainder of said composition constituting iron and impurities, the atomic percentage of Nb plus the atomic percentage of Ti in said steel composition being equal to or greater than the atomic percentage of C plus the atomic percentage of N.

2. The steel of claim 1 wherein the volume ofaustenite in said steel is between 10% and 25%.

3. The steel of claim 1 wherein the composition of said steel, given in percent by weight, consists essentially of:

C, a maximum of 0.10%

Si, a maximum of 1.0%

Mn, a maximum of 2.0%

Cr, from 22% to 28% Ni, from 5% to 8% Mo, from 1% to 2% Nb, from 0.1% to 2.5%

Ti, a maximum of 1.5%

A1, from 0.1% to 2%, and

N, a maximum of 0.1% and the remainder as previously recited.

4. The steel of claim 1 wherein the composition of said steel, given in percent by weight, consists essentially of:

C, a maximum of 0.10%

Si, a maximum of 1.0%

Mn, a maximum of 2.0%

Cr, from 24% to 27% Ni, from 5.5% to 7.5%

Mo, from 1.3% to 1.8%

Nb, from 0.1% to 2.5%

Ti, a maximum of 1.5%

A1, from 0.1% to 2%, and

N, a maximum of 0.1% and the remainder as previously recited.

5. The steel of claim 1 wherein the composition of said steel, given in percent by weight, consists essentially of:

C, a maximum of 0.10%

'Si, a maximum of 1.0%

Mn, a maximum of 2.0%

Cr, from 24% to 27% Ni, from 5.5% to 7.5%

Mo, from 1.3% to 1.8%

Nb, from 0.1% to 2.5%

Ti, a maximum of 1.5%

A1, from 0.5% to 1.0%, and

N, a maximum of 0.1% i and the remainder as previously recited.

6. The steel of claim 1 wherein the composition of said steel, given in percent by weight, consists essentially of:

C, from 0.01% to 0.10%

Si, a maximum of 1.0%

Mn, a maximum of 2.0%

Cr, from 24% to 27% Ni, from 5.5% to 7.5%

Mo, from 1.3% to 1.8%

Nb, from 0.1% to 1.6%

Ti, a maximum of 1.0%

A1, from 0.5% to 1.0%, and

N, a maximum of 0.1%

and the remainder as previously recited. 

1. A CORROSION-RESISTANT, AUSTENITE-FERRITIC STEEL WHICH CONTAINS 10% TI 35% BY VOLUME OF AUSTENITE, SAID STEEL BEING ADAPTED TO ATTAIN AN INCREASE IN STRENGTH BY AGE-HARDENING, WHILE RETAINING GOOD DUCTILITY AND NOTCH-IMPACT RESISTANCE AS WELL AS EXCELLENT RESISTANCE TO INTERCYSTALLINE CORROSION, SAID STEEL CONSISTING ESENTIALLY OF THE FOLLOWING COMPOSITION BY WEIGHT: C, A MIXIMUM OF 0.15% SI, A MIXIMUM OF 2% MN, A MAXIMUM OF 5% CR, FROM 20% TO 30% NI, FROM 4% TO 9% MO, FROM 1% TO 3% NB, FROM 0.1% TO 2.5% TI, A MIXIMUM OF 1.5& AL, FROM 0.1% TO 2%, AND N, 2 MAXIMUM OF 0.15%, THE REMAINDER OF SAID COMPOSITION CONSISTUTING IRON AND IMPURITIES, THE ATOMIC PERCENTAGE OF NB PLUS THE ATOMIC PERCENTAGE OF IT IN SAID STEEL COMPOSITION BEING EQUAL TO OR GREATER THAN THE ATOMIC PERCENTAGE OF C LUS THE ATOMIC PERCENTAGE OF N.
 2. The steel of claim 1 wherein the volume of austenite in said steel is between 10% and 25%.
 3. The steel of claim 1 wherein the composition of said steel, given in percent by weight, consists essentially of: C, a maximum of 0.10% Si, a maximum of 1.0% Mn, a maximum of 2.0% Cr, from 22% to 28% Ni, from 5% to 8% Mo, from 1% to 2% Nb, from 0.1% to 2.5% Ti, a maximum of 1.5% Al, from 0.1% to 2%, and N, a maximum of 0.1% and the remainder as previously recited.
 4. The steel of claim 1 wherein the composition of said steel, given in percent by weight, consists essentially of: C, a maximum of 0.10% Si, a maximum of 1.0% Mn, a maximum of 2.0% Cr, from 24% to 27% Ni, from 5.5% to 7.5% Mo, from 1.3% to 1.8% Nb, from 0.1% to 2.5% Ti, a maximum of 1.5% Al, from 0.1% to 2%, and N, a maximum of 0.1% and the remainder as previously recited.
 5. The steel of claim 1 wherein the composition of said steel, given in percent by weight, consists essentially of: C, a maximum of 0.10% Si, a maximum of 1.0% Mn, a maximum of 2.0% Cr, from 24% to 27% Ni, from 5.5% to 7.5% Mo, from 1.3% to 1.8% Nb, from 0.1% to 2.5% Ti, a maximum of 1.5% Al, from 0.5% to 1.0%, and N, a maximum of 0.1% and the remainder as previously recited.
 6. The steel of claim 1 wherein the composition of said steel, given in percent by weight, consists essentially of: C, from 0.01% to 0.10% Si, a maximum of 1.0% Mn, a maximum of 2.0% Cr, from 24% to 27% Ni, from 5.5% to 7.5% Mo, from 1.3% to 1.8% Nb, from 0.1% to 1.6% Ti, a maximum of 1.0% Al, from 0.5% to 1.0%, and N, a maximum of 0.1% and the remainder as previously recited. 